Because of erythropoietin's central role in red cell formation and its possible role in regulation of hemoglobin synthesis, we have embarked on a program to attempt its purification and also to obtain specific antibodies to this substance. In addition, we hope to use similar techniques to purify and obtain antibodies to other growth factors important during erythropoiesis. To date, partial purification of erythropoietin has been accomplished using a wheat germ agglutinin affinity column and this preparation has been used to immunize mice. Spleen cells from hyperimmunized animals will be fused to myeloma cells to form hybridoma cells, some of which may produce monospecific antibodies to erythropoietin. An assay to detect these antibody producing clones of hybridoma cells has been developed. Removal of erythropoietin present in culture media, by antibody bound to an insoluble matrix, is detected by subsequent lack of erythroid colony growth in the culture media.